The present invention generally relates to a wheel speed detecting apparatus and more particularly, to a wheel speed detecting apparatus suitable for an antiskid control device, which is capable of detecting wheel speed accurately even when setting of change gear ratio of a transmission mechanism or wheel radius has been changed.
Generally, in an antiskid brake system for a motor vehicle, skid symptom is detected by comparing wheel speed and vehicle body speed with each other. Thus, various wheel speed detecting apparatuses used for the antiskid brake system have been so far provided. Usually, the wheel speed detecting apparatuses of this kind are constituted by wheel speed sensors and a controller. Each of the wheel speed sensors is fixed to a rotary shaft coupled with wheels and includes a geared rotor rotatable with the rotary shaft and a pickup sensor for detecting each of teeth of the rotating rotor and outputting pulse signals, which is provided so as to confront the rotor. The controller calculates wheel speed from the pulse signals inputted from the wheel speed sensors.
An antiskid brake system for a four-wheel motor vehicle of rear wheel drive shown in FIG. 1 includes first and second wheel speed sensors S.sub.1 and S.sub.2 for outputting pulse signals in response to rotation of a rotary shaft 1A for a front left wheel FL and a rotary shaft 1B for a front right wheel FR, respectively and a third wheel speed sensor S.sub.3 for outputting pulse signals in response to rotation of a propeller (drive or propulsion) shaft 2. A controller 4 calculates rotational angular velocities SA.sub.1 and SA.sub.2 of the rotary shafts 1A and 1B for the front left and right wheels FL and FR from the pulse signals of the first and second wheel speed sensors S.sub.1 and S.sub.2 and further calculates wheel speeds SW.sub.1 and SW.sub.2 of the front left and right wheels FL and FR from the rotational angular velocities SA.sub.1 and SA.sub.2 and radius of the front left and right wheels FL and FR (wheel radius).
Meanwhile, the controller 4 calculates a rotational angular velocity SA.sub.3 of the propeller shaft 2 from the pulse signals of the third wheel speed sensor S.sub.3 and further calculates an average wheel speed of rear left and right wheels RL and RR (average rear wheel speed) from the rotational angular velocity SA.sub.3 and a conversion factor indicative of change gear ratio of a differential gear 5 and wheel radius.
However, change gear ratio of the differential gear 5 varies according to models of motor vehicles, etc. and thus, the conversion factor is required to be changed accordingly. Meanwhile, also when wheel radius changes upon replacement of wheels, etc., the conversion factor should be changed accordingly. Therefore, in case change gear ratio of the differential gear 5 or wheel radius changes, the controller 4 should be replaced by another one.
In order to eliminate such a drawback of the known wheel speed detecting apparatus, Japanese Patent Laid-Open Publication No. 5-16774 (1993) proposes a wheel speed detecting apparatus in which a selector switch is provided in a controller such that the conversion factor can be set selectively in accordance with change gear ratio and wheel radius. However, in this prior art wheel speed detecting apparatus, since the selector switch is required to be provided in the controller, production cost of the controller rises accordingly. Meanwhile, if the selector switch malfunctions, wheel speed is not calculated properly in accordance with each change gear ratio, thereby resulting in deterioration of control performance of the antiskid brake system. Furthermore, when an operator inadvertently fails to change setting of the conversion factor by using the selector switch in spite of the fact that change gear ratio of the differential gear or wheel radius has been changed, the wheel speed detecting apparatus cannot be controlled correctly.